In recent times several techniques for using dual pumps or dual acting pumping systems in well completion and production techniques have been disclosed and used. In these techniques, the well borehole annulus (cased) in a production zone producing oil and water has been used to gravity separate the produced oil from the produced water. Dual acting pumps or dual pumps are then used to pump the produced oil to the surface via production tubing and to reinject the produced water into a disposal zone, either above or below, the production zone. The heavier produced water settles to the bottom of the casing/tubing annulus and a pump inlet near the bottom of the production zone picks up primarily produced water, if placed there. Similarly, a second pump inlet placed a distance uphole, around or above the production perforation, picks up primarily oil, if placed there. If the water is re-injected below the production zone, usually a packer is set in the casing below the production zone and the water simply pumped below this packer to re-enter the disposal zone through disposal perforations located below the packer. If the disposal zone is located above the production zone in the well, then the production zone must be isolated, interior to the casing, by a packer placed between the disposal zone and the production zone in the casing. This packer must be penetrated by both the production tubing string and a tubing string carrying the produced water to the disposal zone perforations. The use of such dual tubing strings becomes difficult in small diameter well casing such as 5 inch ID (inside diameter) casing, since standard production tubing has an (outer diameter) OD of # inches. It would be desirable therefore to avoid the use of dual tubing strings, if possible.
If the produced crude oil in a well has a high viscosity (such as those heavy crudes in many parts of the U.S. and the world), say up to 110,000 (centiPoise) cP, then friction losses in pumping such viscous crudes through tubing or pipe can become very significant. Such friction losses (of pumping energy) are due to the shearing stresses between the pipe or tubing wall and the fluid being transported. This can cause significant pressure gradients along the pipe or tubing. In extremely viscous crude production such pressure gradients cause large energy losses in pumping systems, both within the well and in surface pipelines. Typically, attempts to lower the crude oil viscosity by the use of diluents or through heating have been used. It has also been proposed to form oil/water emulsions by using re-injected produced water or such water in an oil-in-water emulsion stabilized by chemical agents. All such techniques have the disadvantage of being expensive and energy consumptive. It would therefore be desirable to provide economical, simple techniques for moving heavy or viscous crudes in pipe or tubing flow in a well without resorting to dual tubing strings or to heat or chemical injection to lower viscosity of the crude.
It has recently been proposed to use the flow regime, or mode known as core annular flow to improve the portability of heavy crude in surface pipelines. In core annular flow a less viscous immersible fluid, such as water, is introduced into the crude flow in a particular geometrical manner to form a sheath or layer of flow next to the tubing or pipe wall with the heavy crude moving interiorly to the annular flow of the less viscous fluid. The core annular flow mode greatly reduces the pressure required to move viscous crude in a pipe or tubing string.